1. Field of the Invention
The present invention relates to a television tuner.
2. Description of the Related Art
A conventional television tuner will be described with reference to FIG. 2. In FIG. 2, an interstage double-tuned circuit 53 (hereinafter, referred to as a double-tuned circuit) is provided between a high-frequency amplifier 51 and a mixer 52. A primary tuning circuit 53a includes a high-band tuning coil 54 and a low-band tuning coil 55 connected in series at one end of each coil, and a varactor diode 56 connected in parallel to the high-band tuning coil 54 and the low-band tuning coil 55 connected in series. An anode of the varactor diode 56 is grounded, and a cathode thereof is connected to the other end of the high-band tuning coil 54 through a DC-blocking capacitor 57 interposed therebetween.
In a similar manner, a secondary tuning circuit 53b includes a high-band tuning coil 58 and a low-band tuning coil 59 connected in series at one end of each coil, and a varactor diode 60 connected in parallel to the high-band tuning coil 58 and the low-band tuning coil 59 connected in series. An anode of the varactor diode 60 is grounded, and a cathode thereof is connected to the other end of the high-band tuning coil 58 through a DC-blocking capacitor 61 interposed therebetween.
Further, the other ends of the low-band tuning coils 55 and 59 are connected to one end of a coupling coil 62, and the other end of the coupling coil 62 is grounded through a DC-blocking capacitor 63.
Furthermore, a connection point of the high-band tuning coil 54 and the low-band tuning coil 55 in the primary tuning circuit 53a is connected to a cathode of a switching diode 64, and an anode thereof is grounded through a DC-blocking capacitor 65. Similarly, a connection point of the high-band tuning coil 58 and the low-band tuning coil 59 in the secondary tuning circuit 53b is connected to a cathode of a switching diode 66, and an anode thereof is grounded through a DC-blocking capacitor 67.
In addition, the anode of the switching diode 64 is connected to a high-band switching terminal 69 through a resistor 68, and similarly, the anode of the switching diode 66 is connected to the high-band switching terminal 69 through a resistor 70.
Moreover, a connection point of a coupling capacitor 62 and the DC-blocking capacitor 63 is connected to a low-band switching terminal 72 through a resistor 71. Also, a tuning voltage is applied to the cathode of the varactor diode 56 from a tuning voltage terminal 73 through a resistor 74, and similarly, the tuning voltage is applied to the cathode of the varactor diode 60 from the tuning voltage terminal 73 through a resistor 75.
A peaking circuit 76 is provided between the double-tuned circuit 53 and the mixer 52. The peaking circuit 76 is DC-blocked from the double-tuned circuit 53 by a DC-blocking capacitor 77 and also DC-blocked from the mixer 52 by a DC-blocking capacitor 78.
The peaking circuit 76 is composed of a peaking coil 79, a switching diode 80 and a DC-blocking capacitor 81 connected to each other in series. The peaking coil 79 is connected to the DC-blocking capacitors 77 and 78, and one end of the DC-blocking capacitor 81 is grounded. Also, a cathode of the switching diode 80 is connected to the low-band switching terminal 72 through a resistor 82, and an anode thereof is connected to the high-band switching terminal 69 through a resistor 83. The inductance of the peaking coil 79 and an input capacitance 84 of the mixer 52 are set to cooperate to resonate with the frequency of a lower-band television signal within the range of high-band television signals.
In the above-mentioned configuration, when high-band television signals included in a VHF band are received, a current passes through both of the switching diodes 64 and 66 by a switching voltage applied to the high-band switching terminal 69, so that the double-tuned circuit 53 is in a state in which the high-band television signals are received. The switching voltage applied to the high-band switching terminal 69 is also applied to the anode of the switching diode 80, so that the switching diode 80 is also in an electrical connection state. As a result, a parallel resonating circuit is constructed by the peaking coil 79 and the input capacitance 84 of the mixer 52.
Further, by the parallel resonating circuit constructed by the peaking coil 79 and the input capacitance 84 of the mixer 52, the gain of a lower-band television signal within the range of the high-band television signals is increased.
In addition, by the tuning voltage applied to the tuning voltage terminal 73, the double-tuned circuit 53 is tuned to a desired frequency (for example, see Japanese Unexamined Patent Application Publication No. 11-275478 (FIG. 2)).
However, the television tuner has a complicated configuration because the interstage tuning circuit is a band switching type. Also, since the peaking circuit can not be constructed when low-band television signals are received, there is a problem in that a gain deviation in the low band cannot be solved.